The present invention relates generally to radio communication systems, and more particularly to a method for transmitting wideband signals via a communications system adapted for transmitting narrow-band signals.
Radio communication systems rely on modulating carrier frequencies in a finite portion of the electromagnetic spectrum to wirelessly transmit and receive signals. Modulation can be performed on the amplitude, frequency, and/or phase of the carrier frequency to separate the signal from unwanted noise. The signals typically convey information such as voice, video, and computer data to and from transceiving devices such as cellular base stations, cellular subscriber units, and personal computers.
The portion of the electromagnetic spectrum occupied by a particular transmission or communication system (i.e. bandwidth) may be wide or narrow. Wideband signals can be used to transmit large amounts of data in a relatively short period of time. For example, large computer data files and real-time video could benefit from a wideband signal. Narrow-band signals can be used to conserve the electromagnetic spectrum when transmitting signals with more modest requirements. For example, base stations and cellular subscriber units in most conventional cellular communication systems transmit and receive voice signals using a relatively narrow-band signal.
The amount of usable electromagnetic spectrum is limited by technology, environment, and cost. Extremely high frequency signals require expensive transceiving equipment. Accordingly, communication systems benefit by sharing desirable frequencies. Well-known multiple access techniques, such as code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA) can be used by a communication system to share the electromagnetic spectrum available to that system. However, these techniques require the signals to be of substantially the same bandwidth. For example, a cellular base station may transmit to a plurality of cellular subscriber units by dividing a portion of the spectrum (e.g., 869 MHz-894 MHz) into a plurality of relatively narrow-band channels (e.g., 30 kHz). Similarly, a satellite communication system may transmit to a plurality of ground stations by dividing a portion of the spectrum (e.g., 3700 MHz-4200 MHz) into a plurality of relatively wideband channels (e.g., 36 MHz). The narrow-band signals and wideband signals occupy distinct portions of the electromagnetic spectrum in order to avoid interfering with each other.
Prior art approaches to bandwidth utilization suffer from certain drawbacks. For instance, prior art approaches do not allow wideband signals to occupy excess capacity in a narrow band system or narrow-band signals to occupy excess capacity in a wide band system. Further, prior art approaches require new communication systems infrastructure (e.g., base stations) to support new types of signals (i.e., signals using different bandwidths).
In accordance with a first aspect of the invention, a method of transmitting signals having a first bandwidth via a communication system adapted for communicating signals of a second bandwidth, wherein the second bandwidth is narrower than the first bandwidth , is provided. The method comprises the steps of modulating a first data signal with a particular orthogonal code to produce a first spreaded signal having the first bandwidth and transmitting simultaneously the first spreaded signal and the signals of the second bandwidth so that the first bandwidth overlays the second bandwidth. Preferably, multiple users may share the first bandwidth using a time division multiple access protocol.
In certain embodiments, the method further comprises a step of transmitting a pilot signal during transmission of the first bandwidth signals. Preferably, the pilot signal is terminated approximately when the transmission of the first bandwidth signals are terminated. Further, a step of transmitting control data for the first bandwidth signals on a control channel shared by the first bandwidth users may optionally be performed using a time division multiple access protocol.
In a preferred embodiment, the method further comprises a step of encoding the first bandwidth signals with forward error correction codes. In such an instance, the forward error correction codes may optionally be turbo codes. Preferably, the first bandwidth signals originate at a base station and are addressed to a predefined receiver. Also preferably, the second bandwidth signals comprises voice signals and the receivers of the second bandwidth signals comprise cellular subscriber units.
In accordance with another aspect of the invention, a method of adapting a data transmission rate, in response to a data error rate and a transmitter power level, to increase transmission range is provided. The method comprises the steps of increasing the transmitter power level in response to the data error rate and determining a predefined threshold for the transmitter power level. The method further comprises the step of decreasing the data transmission rate in response to the transmitter power level being substantially equal to the predefined threshold, until a predefined error rate is achieved.
In some embodiments, the predefined threshold is a maximum power level or a predefined percentage of a maximum power level. Preferably, the predefined error rate is adjusted according to a type of data being transmitted. In such an instance, when the type of data being transmitted is substantially encoded voice signals, a first error rate is used and when the type of data being transmitted is substantially computer data, a second error rate is used.
In accordance with yet another aspect of the invention, a method of adapting a data transmission rate, in response to a data error rate and a transmitter power level, to increase data transmission rate is provided. The method comprises the steps of decreasing the transmitter power level in response to the data error rate and determining a predefined threshold for the transmitter power level. The method further comprises the step of increasing the data transmission rate in response to the transmitter power level being substantially equal to the predefined threshold, until a predefined error rate is achieved.
In some embodiments, the predefined threshold is a minimum power level or a predefined percentage of a maximum power level. Preferably, the predefined error rate is adjusted according to a type of data being transmitted. In such an instance, when the type of data being transmitted is substantially encoded voice signals, a first error rate is used and when the type of data being transmitted is substantially computer data, a second error rate is used.
In accordance with still another aspect of the invention, a method of transmitting a message intended for transmission on a first bandwidth via a communication system adapted for communicating signals of a second bandwidth, wherein the second bandwidth is narrower than the first bandwidth is provided. The method comprises the steps of determining a length for the message and determining a predefined threshold for message length. The method further comprises the step of transmitting the message using the first bandwidth signal via an access channel when the length is less than the predefined threshold.